ROSTRUM, P.RANCHIAL-ARCHES, FINS. 



129 



nasal septum (usually its base) is probably represented in the element which 

 Schauinsland has figured as sp in his plate xvii, fios. 124, 126. However, in 

 the Chima?roid the rostral supports (/-'and r" ) later developed into long and 

 separately jointed elements. Quite doubtful, on the other hand, are the homo- 

 logues of the paired dorsal elements in the selachian rostrum, those figured, 

 c. g., by Kitchen Parker in Trans. Zool. Soc, vol. x, plate xxxviii, fig. i, as btr; 

 they are possibly the homologues of Schauinsland's elements ,s- in the figures quoted. 

 Equally doubtful is the more dorsal azygous element (Schauinsland's rV v. the 

 present fig. 1 1 i ), which folds forward and becomes a main support of the produced 

 snout in Callorhvnchus; it certainly finds no homologue in sharks, and in view of the 

 history of the frontal clasping organ in Chimseroids {v. figs. 132-137) I am inclined 

 to interpret it as an element, /. r., a fin support, transjiosed from a hinder position,* 

 a view which is the less difficult to accept when one considers the metamorphosis 

 to which the head roof has been subjected by the precocious growth of the eyes. 



M 



BH B 



Figs. 1 10 and 



'V b'H B'B-'B-'B'B^ 

 Skull and branchial arches of Shark and Chimaeroid compared. 



i?I-i?'>, Branchial arches: i? //, basihyal : 6^^, basis trabecularum (Kitchen Parker) : C, copula : C/?, ceralobranchial : AV>', Epibranchial; 

 ///>', hypobranchial ; /'. " anteriormosl lip cartilage " (Kitchen Parker); 3A mandible ; -P-S, pharyngobranchiaL 



The history of the fins and their supports, finally, gives additional evidence as 

 to the modified nature of later Chimasroid development. We may comment, for 

 example, upon the appearance of lobate dorsal fins, the anterior with its spine, at 

 an early period, and the prominence of the paired fins, the pectoral, for example, 

 having at one time a greater proportional size than in the adult. We observe also 

 the i)recocious appearance of the mixipterygia and the antero-pelvic appendages 

 (note especially plate ix, fig. 50*"; also fig. 90, and Schauinsland's Taf. xvi, figs. 

 120 and 125), a well-marked character which in such early embryos can hardly be 

 regarded as primitive. Nor is the plan of development of the paired fins to be 

 looked upon as yielding any evidence in favor of Gegenbaur's archipterygium 

 theory. Thus, the pectoral, for example, appears not as a lobate organ, contracted, 

 shortly to bud out radial structures, but as a lappet of a lateral fold which shows in 

 the early stages distinct metameral elements {cf. especially plate viir, fig. 49, and 

 Schauinsland's Taf. xxiv, fig. 174).* The paired fins, in short, develop like those of 



*Thi5 translocation ot anterior fin-rays is by no means uncommon, associated, too, with change of function, e.g., 

 Lophius, Antennarius, etc. Even the sucking disc of Remora might here be cited. 



